1. Field of the Invention
The present invention relates to a steam boiler apparatus, and more particularly to a steam boiler apparatus designed to supply feed-water for use as boiler water, from a feed-water supply line made of metal, and heat the boiler water in a steam boiler to generate steam.
2. Description of the Background Art
A steam boiler apparatus is equipped with a steam boiler and a feed-water supply system, and designed to supply feed-water for use as boiler water, from the feed-water supply system, and heat the boiler water in the steam boiler to generate steam. In some cases, the steam boiler apparatus is provided with a preheater for the feed-water, in order to suppress energy consumption due to heating of the boiler water in the steam boiler. The preheater is operable to heat a feed-water supply pipe of the feed-water supply system by use of exhaust heat from the steam boiler to thereby preliminarily heat feed-water to be supplied to the steam boiler.
In the above steam boiler apparatus, each of the feed-water supply pipe and a heat transfer tube for heating the boiler water in the steam boiler is made of metal, which means that each of the feed-water supply pipe and the heat transfer tube is placed under conditions where corrosion is likely to develop due to influence of the feed-water or the boiler water. Particularly, the feed-water supply pipe is always in contact with the feed-water, so that corrosion is likely to develop in a portion thereof to be heated by the preheater.
Typically, a water source of feed-water for the steam boiler apparatus is tap water or groundwater which contains an alkaline component (hydrogen carbonate and carbonate) as a natural component. Therefore, in the steam boiler supplied with such feed-water, the alkaline component is thermally decomposed to form a hydroxide, so that a pH value of boiler water can be increased to suppress corrosion of the heat transfer tube. However, in the feed-water supply pipe, even if the feet-water is subjected to heating by the preheater, a temperature of the feed-water is increased up to only about 120° C. at the highest. Thus, it is difficult to promote thermal decomposition of the alkaline component, i.e., to sufficiently increase a pH value of the feed-water as in the boiler water, so that the feed-water supply pipe is more likely to suffer development of corrosion than the heat transfer tube.
In view of this, JP 2006-275410A (hereinafter referred to as “Patent Document 1”) proposes a technique of, in a feed-water supply system, allowing a part of boiler water having a pH value increased based on formation of a hydroxide to be returned or recirculated to an upstream side of a preheater and mixed with feed-water. In this technique, the boiler water is mixed with feed-water to increase a pH value of the feed-water, so that the feed-water becomes less likely to cause corrosion of a feed-water supply pipe in the preheater.
However, a change in pH value of the feed-water based on the mixing of the boiler water with the feed-water is influenced by a concentration of the alkaline component of the feed-water. Specifically, in cases where the alkaline component concentration is relatively low, the pH value of the feed-water is easily increased by mixing the boiler water with the feed-water. Conversely, in cases where the alkaline component concentration is relatively high, even if the boiler water having a high pH value is mixed with the feed-water, the pH value of the feed-water often shows only a slight increase, due to a buffering ability of the alkaline component. The alkaline component concentration of the feed-water varies depending on regionality of tap water or groundwater for use as a water source, and has an extremely high value in some cases. Thus, in a region where it is necessary to use such a water source as feed-water, it is difficult to suppress corrosion of a feed-water supply pipe in a preheater by the technique disclosed in the Patent Document 1.